Light coupling devices have a semiconductor light emitting device to emit a light signal with a wavelength range from red light to infrared light, and a light receiving device such as a Si-light receiving device to convert the light signal into an electric signal and output the electric signal. The light coupling devices enable signals to be transmitted with electrical insulation between an input terminal and an output terminal.
The light coupling devices to transmit high-speed signals are increasingly expanding their application in industrial instruments, communication devices, and the like.
Reduction of a time constant of the light receiving device, or a circuit configured to detect change of an output to speed up changeover of an output voltage enables the light receiving device to be driven at high speed.
On the other hand, since the light emitting device emits light by applying a forward voltage, a junction capacitance of the light emitting device is larger than that of the light receiving device, and thereby it is hard to improve the response speed of the light emitting device.